1. Field of the Invention
The present invention relates to an optical pickup device, an optical recording and reproducing apparatus and a gap detection method suitable for use with an optical recording and reproducing apparatus which carries out at least one of recording and reproduction of signals by using near field light.
2. Description of the Related Art
In recent years, in accordance with an increasing request for increasing a recording density and a storage capacity, various methods have so far been proposed to record and reproduce an optical recording medium by irradiating the optical recording medium with near field light through a condensing lens having a large numerical aperture. As a condensing lens using such near field light, there has been proposed so far a condensing lens which can realize a numerical aperture greater than 1 by a combination of a solid immersion lens (hereinafter simply referred to as an “SIL”) and an optical lens such as an aspherical lens.
In an optical recording and reproducing apparatus using near field light, a distance between an optical head and an end face of a tip end side lens such as an SIL and a signal recording surface of an optical recording medium is selected to be approximately ¼ of a wavelength of incident light, and it is requested that such distance should be selected to be lower than approximately 100 nm in a wavelength range of a light source which is now commercially available.
For this reason, even when such SIL is inclined very slightly, that is, such SIL has a so-called tilt angle, there is a possibility that the SIL will hit against the recording surface of the optical recording medium easily. If the SIL hits against the recording surface of the optical recording medium, then a disadvantage will occur, in which the recording surface of the optical recording medium will be damaged. As a result, there is a risk that the optical recording medium cannot be recorded or reproduced normally.
Relative to the near field optical system using near field light, in an optical pickup device with an ordinary optical system that does not use near field light, that is, so-called far-field optical system, there has been proposed a method in which tilt detection laser light is separated from laser light by a diffraction grating, focusing signals at respective beam spots are calculated by a suitable method such as an astigmatism method and a knife edge method and tilts being corrected so that calculated focusing signals may become equal to each other (see Cited Patent Reference 1, for example).
Also, in an ordinary optical pickup device which does not use near field light similarly, there has been proposed a method in which tilt detection laser light is separated from laser light, a tilt quantity being calculated from a plurality of beam spots by a tilt sensor (see Cited Patent Reference 2, for example).
[Cited Patent Reference 1]: Official Gazette of Japanese laid-open patent application No. 2002-197699
[Cited Patent Reference 2]: Official Gazette of Japanese laid-open patent application No. 2000-155969
However, the optical pickup device using near field light may not detect a tilt or a gap by using the focusing signal according to the above-mentioned related-art methods.
Further, when an external tilt sensor or gap sensor is in use, an arrangement of an optical pickup device becomes complex and hence a problem arises, in which a cost of the optical pickup device will be increased unavoidably. Alternatively, since the position at which a tilt or gap is detected by the external sensor and the recording and reproducing position are different from each other, phases of the tilt detection position or the gap detection position and the recording and reproducing position should be matched separately.